It is generally known to use Coriolis effect mass flow meters to measure mass flow and other information for materials flowing through a conduit in the flow meter. Exemplary Coriolis flow meters are disclosed in U.S. Pat. Nos. 4,109,524, 4,491,025, and Re. 31,450 all to J. E. Smith et al. These flow meters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flow meter has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at a preferred mode.
Material flows into the flow meter from a connected pipeline on the inlet side of the flow meter, is directed through the conduit(s), and exits the flow meter through the outlet side of the flow meter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flow meter, a driving force applied to the conduit(s) causes all points along the conduit(s) to oscillate with identical phase or small initial fixed phase offset, which can be corrected. As material begins to flow through the flow meter, Coriolis forces cause each point along the conduit(s) to have a different phase. For example, the phase at the inlet end of the flow meter lags the phase at the centralized driver position, while the phase at the outlet leads the phase at the centralized driver position. Pick-off sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-off sensors are processed to determine the phase difference between the pick-off sensors. The phase difference between the two or more pick-off sensors is proportional to the mass flow rate of material flowing through the conduit(s).
Meter electronics connected to the driver generates a drive signal to operate the driver and determines a mass flow rate and other properties of a material from signals received from the pick-off sensors. The driver may comprise one of many well known arrangements; however, a magnet and an opposing drive coil has received great success in the flow meter industry. An alternating current is passed to the drive coil for vibrating the conduit(s) at a desired flow tube amplitude and frequency. It is also known in the art to provide the pick-off sensors as a magnet and coil arrangement very similar to the driver arrangement. However, while the driver receives a current which induces a motion, the pick-off sensors can use the motion provided by the driver to induce a voltage.
The magnitude of the time delay measured by the pick-off sensors is very small; often measured in nanoseconds. Therefore, it is necessary to have the transducer output be very accurate. Transducer accuracy may be compromised by nonlinearities and asymmetries in the meter structure or from motion arising from extraneous forces. For example, a Coriolis mass flow meter having unbalanced components can vibrate its case, flanges, and the pipeline at the drive frequency of the meter. This vibration perturbs the time delay signal in an amount that depends on the rigidity of the mount. Since the rigidity of the mount is generally unknown and can change over time and temperature, the effects of the unbalanced components generally cannot be compensated and may significantly affect meter performance. The effects of these unbalanced vibrations and mounting variations can be reduced by using flow meter designs that are balanced and by using signal processing techniques to compensate for unwanted component motion.
Typical dual tube Coriolis flow meter designs split the flow of material into two streams using manifolds and send the two streams of material into the two separate flow tubes. The two tubes are typically symmetrical in shape and mounted parallel to one-another. The two tubes typically vibrate at the same frequency but in opposite phase. Because the tubes are symmetrical and vibrated opposite each other, the vibrations typically cancel out where the two tubes are joined. This creates a balanced flow meter (i.e., little or no vibration of the meter at the manifolds). A change in density in the material flowing through the two tubes changes the mass of both tubes substantially equally and therefore, the two tubes remain balanced across a wide range of material densities.
There are certain applications where dual tube meters are not wanted due to the pressure drop and/or plugging issues created by the manifold, for example. In these situations, a single tube meter is often desirable. A problem with single tube Coriolis flow meters is that they can become imbalanced with changing fluid densities. As the density of the fluid flowing through the flow meter changes, the center of mass of the flow meter also changes. This imbalance can have adverse effects on the meter's performance and reliability.
Therefore, there is a need in the art for a single tube flow meter that is capable of remaining balanced over a wide range of material densities. There are a number of prior art attempts, none of which have provided satisfactory results over a wide range of fluid densities. For example, some single tube flow meters have incorporated a separate counter-balance bar. While such a solution may provide acceptable results for limited fluid density ranges, if both the driver components and pick-off components are not connected to the balance bar, the solution is incomplete. Another prior art solution has been to reference the flow tube's vibration against a rigid mounting plate. Although this provides adequate results in an ideal situation, it is often difficult to create an absolutely stable mounting plate. Therefore, external vibrations that are translated to the mounting plate can adversely affect the meter's performance.
Another prior art solution is introduced in U.S. Pat. No. 6,666,098, which discloses the use of a counter tube that runs parallel to the flow tube. One potential problem with this design lies in the fact that the flow tube and the counter tube must be made of substantially the same material and have approximately the same mass distribution. The same mass distribution is essential to maintain a balanced system in this prior art approach. Furthermore, because the flow tube and the counter tube extend parallel to one another and are therefore approximately the same length, the two should be made of the same material. If the flow tube and counter tube comprise different materials having different thermal coefficients of expansion, changes in temperature may create axial stresses in the flow tube resulting in erroneous measurements. While this may not seem to be a significant problem, it should be appreciated that in many circumstances it is desirable to provide a flow tube having a relatively low thermal coefficient of expansion, such as titanium or zirconium. However, such materials are often expensive. By requiring both the flow tube and the counter tube to be formed from an expensive material, the cost of manufacturing the flow meter is significantly increased.
The present invention overcomes this and other problems and an advance in the art is achieved. It should be appreciated however, that while the present invention overcomes difficulties that are particularly prevalent with single tube designs, the invention is equally applicable to dual tube meters. Although the description that follows is directed mainly towards Coriolis flow meters, it should be appreciated that the invention is equally applicable to other vibrating structures that lack the measurement capabilities of Coriolis flow meters, such as vibrating densitometers.